Quantum chemical identification of blue and red forms of protonated pheophytin-a dianion

Quantum mechanics/molecular mechanics (QM/MM) calculations were employed so as to determine the most stable structures of protonated forms of Pheophytin-a (Pheo) dianion in DMF. The protonated forms include PheoH− and PheoH2. Electrostatic potential-derived charges (ESP charges) employing the CHelpG scheme were obtained for Pheo2− and for possible blue and red forms of PheoH− by ROB3LYP methodology using 6-31G(d) basis. The ESP charges and the most stable structures obtained by QM/MM calculations provide a clue to the probable site of protonation in Pheo2− and PheoH−. A total of 18 different possible structures were investigated. From these calculations, we identify the structures of the blue and red forms of PheoH− and PheoH2. Furthermore, density functional treatment (DFT) along with dielectric polarizable continuum model (DPCM) calculations using 6-311+G(2d,2p) basis were done to determine the absolute free energy of reduction of Pheo to PheoH− and PheoH2 in DMF. The calculated mid-point reduction potentials relative to the standard hydrogen electrode are −0.63 V (blue), −0.71 V (red) for reduction of Pheo to PheoH−, and 0.15 V (blue), 0.11 V (red) for reduction to PheoH2. The calculated numbers are in good agreement with observed values, −0.68, −0.72, 0.15 and 0.05 V, respectively.